This invention relates to a process for the recovery of pure methylal from mixtures of methylal and methanol, which will often contain substances such as water, formaldehyde, methyl formate, as well as others. More specifically, the invention especially relates to a process for obtaining pure methylal from mixtures produced in the synthesis of methylal from a formalin solution and methanol.
It is known in the prior art to produce an approximately 8% strength methanol-containing methylal mixture in an industrial process employing iron (III) chloride as a catalyst, and requiring no specialized technical equipment (see Houben Weyl "Methoden der organ. Chemie" [Methods of Organic Chemistry], vol. VI/3, Oxygen Compounds 1, Part 3 [1965]: 207). In this type process iron (III) sulfate is preferably used in place of iron (III) chloride to avoid chlorinated toxic by-products in the production of the methylal mixture.
It is desirable to recover substantially pure methylal from these mixtures. However, this cannot be readily accomplished by means of a simple rectification because of the existence of an azeotrope. In order to reduce the methanol concentration below 8% various alternative physical or chemical processes have been recommended for the separation of these amounts of methanol.
One such method is taught in Volkov and Ivanov (Vysokomolekul. Soedin 8 (8): 1459-61 [1966]) and Vinokurov (Nauch. Doklady Vysskei Shkoly. Lesoinzhener. Delo. 1958 No. 4: 193-195) wherein the methylal is purified by chemically reacting the methanol present with metallic sodium. Another prior art process teaches that the methanol can also be removed by extraction with concentrated aqueous calcium chloride solution, and by subsequent drying of the methylal (Ullmanns Encyclopaedie der technischen Chemie [Ullmann's Encyclopedia of Technical Chemistry] 3rd ed, vol. 3: 15). Still other known prior art processes for the separation of methanol and methylal include extractive distillation with various extraction agents including among others, water (German Pat. No. 1,002,305 and U.S. Pat. No. 2,545,889), aqueous alkaline solutions (U.S. Pat. No. 2,990,340, Vinokurov, Gidroliz. i Lesokhim. Prom. 12, No. 5: 4-6 [1959]), ethylene glycol (German Pat. No. 1,127,339), paraformaldehyde (Vinokurov, Gidroliz. i Lesokhim. Prom. 12, No. 5: 4-6 [1959]), as well as dimethylformamide (German Pat. No. 1,172,677).
Still another prior art method teaches the chemical reaction of methanol with excess aldehyde to obtain the acetal, i.e., methylal, (Russian Pat. No. 121,443). Likewise, Vinokurov (Izvest. Vysshikh Ucheb. Zavedenii, Lesnoi Zhur. 2, No. 2: 155-159 [1959]) compares three different methods for the separation of methanol and methylal. More specifically, the methods compared are the reaction of methanol with paraformaldehyde, the treatment of the azeotropic mixture of methanol and methylal with sodium hydroxide, and the distillation, as well as rectification, in the presence of an excess of formalin solution.
All of these disclosed processes serve to obtain higher purities in methylal yield by reducing methanol concentration but suffer the disadvantage that all require the use of additional materials. As a result these processes are very expensive and, in part, obtain relatively poor yields of methylal because of the involvement of some of the methylal in the reactions.
It is furthermore known to break an azeotrope by conducting a two step distillation at two different pressures. However, these are systems which have such advantageous properties, that the rectification under different pressures is better than the other above-discussed processes for the separation of the components; these systems are very rare. So it is taught in "Elements of Fractional Distillation" 4th ed. Robinson and Gilliland at page 212, that in view of the high heat consumption and the large number of plates required, other methods are more economical. Moreover, it is an exceptional advantage of this invention to the prior art, that not only is the azeotrope separated, but also the by-products such as methyl formate are removed without additional expense.